diff --git a/esphome/components/web_server/__init__.py b/esphome/components/web_server/__init__.py
index 2710239dd4..dfdc02158a 100644
--- a/esphome/components/web_server/__init__.py
+++ b/esphome/components/web_server/__init__.py
@@ -363,10 +363,13 @@ def serve_local(config: ConfigType, wifi_config: ConfigType | None) -> bool:
 
 def serve_captive(config: ConfigType, full_config: ConfigType) -> bool:
     """web_server runs its own captive portal while the AP is up: embedded interface plus
-    an access point, unless captive_portal (which owns that role) is configured."""
+    an access point, unless captive_portal (which owns that role) is configured. Only on
+    port 80: the OS captive portal probes and the DHCP portal URI always use port 80, so
+    a portal on another port could never be discovered."""
     wifi_config = full_config.get(CONF_WIFI)
     return (
         "captive_portal" not in full_config
+        and config[CONF_PORT] == 80
         and wifi_config is not None
         and CONF_AP in wifi_config
         and serve_local(config, wifi_config)
@@ -391,6 +394,18 @@ def _final_validate_ap_mode(config: ConfigType) -> None:
             "blank. Remove 'local: false', or migrate off version 1, so the interface "
             "is embedded in the firmware."
         )
+    elif (
+        wifi_is_ap_only(wifi_config)
+        and serve_local(config, wifi_config)
+        and config[CONF_PORT] != 80
+    ):
+        _LOGGER.warning(
+            "WiFi is AP only and web_server uses port %d. The interface cannot open "
+            "automatically on the access point (captive portal detection only works on "
+            "port 80); open http://192.168.4.1:%d/ manually, or remove 'port:' to use 80.",
+            config[CONF_PORT],
+            config[CONF_PORT],
+        )
 
 
 def _final_validate(config: ConfigType) -> None:
diff --git a/tests/unit_tests/components/test_web_server.py b/tests/unit_tests/components/test_web_server.py
index 5b9541c0aa..e0ce9a6cc9 100644
--- a/tests/unit_tests/components/test_web_server.py
+++ b/tests/unit_tests/components/test_web_server.py
@@ -13,6 +13,7 @@ from esphome.const import (
     CONF_AP,
     CONF_LOCAL,
     CONF_NETWORKS,
+    CONF_PORT,
     CONF_SSID,
     CONF_VERSION,
     CONF_WIFI,
@@ -52,6 +53,8 @@ def test_serve_local(
     [
         # AP only: local is implied, web_server is the captive portal.
         ({CONF_VERSION: 2}, {CONF_WIFI: AP_ONLY}, True),
+        # Captive portal probes only work on port 80.
+        ({CONF_VERSION: 2, CONF_PORT: 8080}, {CONF_WIFI: AP_ONLY}, False),
         # AP fallback needs an explicit local: true to be captive.
         ({CONF_VERSION: 2}, {CONF_WIFI: AP_FALLBACK}, False),
         ({CONF_VERSION: 2, CONF_LOCAL: True}, {CONF_WIFI: AP_FALLBACK}, True),
@@ -67,6 +70,7 @@ def test_serve_local(
 def test_serve_captive(
     web_server_config: dict, full_config: dict, expected: bool
 ) -> None:
+    web_server_config.setdefault(CONF_PORT, 80)
     assert serve_captive(web_server_config, full_config) is expected
 
 
@@ -74,9 +78,9 @@ def test_serve_captive(
     ("web_server_config", "expect_warning"),
     [
         # Explicit local: false on an AP only device: the hosted page will stay blank.
-        ({CONF_VERSION: 2, CONF_LOCAL: False}, True),
+        ({CONF_VERSION: 2, CONF_PORT: 80, CONF_LOCAL: False}, True),
         # Default: embedded and captive, nothing to warn about.
-        ({CONF_VERSION: 2}, False),
+        ({CONF_VERSION: 2, CONF_PORT: 80}, False),
     ],
 )
 def test_final_validate_ap_mode_warns_for_hosted_page(
@@ -89,3 +93,18 @@ def test_final_validate_ap_mode_warns_for_hosted_page(
     finally:
         fv.full_config.reset(token)
     assert ("stays blank" in caplog.text) is expect_warning
+
+
+def test_final_validate_ap_mode_warns_for_non_default_port(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """Captive portal detection needs port 80; other ports get a hint, not captive mode."""
+    config = {CONF_VERSION: 2, CONF_PORT: 8080}
+    token = fv.full_config.set({"web_server": config, CONF_WIFI: AP_ONLY})
+    try:
+        with caplog.at_level(logging.WARNING):
+            _final_validate_ap_mode(config)
+    finally:
+        fv.full_config.reset(token)
+    assert "cannot open automatically" in caplog.text
+    assert "http://192.168.4.1:8080/" in caplog.text